iam a undergrad student from india and i build BEAM robots. i have a big problem which may sound a little silly. how does one produce negative voltage for opamps....? i have constructed a circuit with two voltage regulators (7818 and 7918) producing + and - 18 volts respectively. but this circuit has two grounds also. it does not have a common ground... what do i do now? if i try a common ground the voltages are different. what do i do? please help me...!

You don't want a voltage regulator... you want a voltage inverter... google "ICL7662" to find the spec sheet... the idea is to first use a voltage regulator to get the proper + voltage, then send that regulagted + voltage through an inverter to get a regulated - voltage.

as Mr steve suggested, i have bought a voltage inverter and its very cheap.. i have used it and i got the necessary results.. Thanks Mr steve, Benji, Admin and awally88 for your valuable suggestions...